1. Field of the Invention
The present invention relates to a method for forming a through-hole through the circumferential wall of a metal pipe and a metal pipe worked by the said method. More specifically, the invention relates to metal pipes having many branch holes in the outside circumferential surface, such as a cooling oil jet pipe that mainly serves to cool the cylinder or the piston of an engine and a lubrication oil jet pipe that is used in, for example, a system for supplying (jetting) a lubricant to a sliding portion such as a crank shaft or a cam shaft.
2. Description of the Prior Art
In metal pipes such as a cooling oil jet pipe for cooling the cylinder or the piston of an engine and a lubrication oil jet pipe used in, for example, a system for supplying (jetting) a lubricant to a sliding portion such as a crank shaft or a cam shaft, oil jetting nozzles are attached to the circumferential wall of a pipe body. For example, in conventional metal pipes of the above kind, nozzle pipes or branch pipes are brazed to respective through-holes that are formed through the circumferential wall of a pipe body.
However, the manufacture of metal pipes of the above kind has the following problems.
Common methods of forming through-holes through the circumferential wall of a metal pipe include (1) a method of forming through-holes with a drill, (2) a method of forming through-holes with a punch without inserting a core bar into a pipe, (3) a method of forming through-holes by inserting a core bar into a pipe, and (4) a method of forming through-holes with a laser. In the drilling method (1), chips may go into the inside of a pipe or a burr may be formed at each inside opening portion and much time and labor are needed to remove those chips and burrs. In the punch method (2) without a core bar, a large recess (shear droop) that may be formed at each outside opening portion impairs the shape of a metal pipe. Further, as in the case of method (1), a burr may be formed at each inside opening portion and some operation is needed to remove such burrs. In addition,, depending on the material of a metal pipe itself, particularly when the metal pipe or through-holes to be formed are small in diameter, the punch may be broken and hence it is difficult to form through-holes. In the punch method (3) using a core bar, the hole forming operation requires much time and labor because time and labor are needed for positioning of the core bar and the core bar needs to be moved for each hole forming position. It is necessary to remove punching chips that have dropped inside the core bar. If a burr is formed even at only one inside opening portion, it becomes difficult to pull out the core bar. Further, the hole forming operation cannot be performed on a portion between bent portions formed by bending a metal pipe. The laser method (4) is costly and dangerous to a human body. When a metal pipe is small in diameter, there is a possibility that a portion on the inside surface that is opposed to a through-hole formed may be damaged. Further, a droop may occur around a through-hole formed, or sputters may splash onto the inside surface of a metal pipe.
Where nozzle pipes or branch pipes are brazed to respective through-holes formed by any of the above methods, the brazing area is small because of a short radial length of through-holes, which causes problems that sufficient brazing strength is not secured and that the attachment direction of the nozzle pipes or the branch pipes becomes erroneous.